A method of pre-treating a mask layer prior to etching an underlying thin film is described. A thin film, such as a dielectric film, is etched using plasma that is enhanced with a ballistic electron beam. In order to reduce the loss of pattern definition, such as line edge roughness effects, the mask layer is treated with an oxygen-containing plasma or halogen-containing plasma or a noble gas plasma or a combination of two or more thereof prior to proceeding with the etching process.